Simple systems based on this principle exist in telecommunication networks; they make it possible to considerably reduce the risk of cutting off a connection in case of failure of one of the two channels and are similar to the automatic stand-by systems in the redundant transmission means, but do not generally reduce the error rate of the selected connection. In simple stand-by systems the switching from one digital channel signal to another is effected without any precautions and generally causes an error packet as a result of the loss of a clock, a frame and so on. This is caused by the fact that the two received signals have been transported separately via transmission means and arrive at instants shifted in time.
Consequently, a first technical problem to be solved before the dynamic switching of the signal can be effected correctly consists of bringing the two received signals in phase again. In these conditions it becomes possible to change from a signal of frame TRA to a signal of frame TRB as often as necessary, in response to the respective errors in each frame, without the risk of degrading transmission and to employ the maximum errorless period of time of each digital channel. The problem of correcting the phase of the signals can be solved in a prior-art manner by establishing a feedback loop between the output and the control input of a variable delay circuit inserted in a first information channel, while a control circuit which furthermore receives the same information present on a second channel and shifted in time, is inserted in the loop.
The second technical problem linked with the first problem mentioned above is error detection in the two digital channels and the control of the switch in response to these errors. This error detection is performed in prior-art manner after the signals have been brought into phase again as described, for example, in Patent Application DE-A 38 00 977. However, this document discloses a simple case where there is no discrimination between error levels as regards the gravity of the error and where the probability with which errors simultaneously appear in the bits in the two transmission channels is neglected, which considerably simplifies the switch control.